German Patent DE-AS 1,291,852 (U.S. counterpart being U.S. Letters Pat. No. 2,979,883) pertains to a process for the manufacture of fibers, yarns or films from crystallizable polyester, by the following steps: (a) slow melt spinning, (b) subsequent warm drawing and (c) thermal shrinkage. The thus obtained fibers, yarns or films may be irreversibly lengthened by moderate heat treatment, without having a tension exerted thereon. Once lengthened, these fibers do not return on their original length by cooling and drying.
Moreover, Japanese Application 0 055 268 discloses the cold drawing of a polyester yarn in a draw ratio of 1.05-1.35, while the yarn is being spun at a rate ranging between 4000 and 7000 m/min. Notwithstanding the above references, the literature does not disclose the possibility of subjecting such yarns to aftertreatment processes.